Henneth Annun Reseach Center

Places in Middle-earth

Udûn — Mordor

Description: "To meet [a spur of the Ephel Dúath] there stretched out from the grey and misty northern range of Ered Lithui a long jutting arm; and between the ends there was a narrow gap: Carach Angren, the Isenmouthe, beyond which lay the deep dale of Udûn. In that dale behind the Black Gate were the tunnels and deep armouries that the servants of Mordor had made for the defence of the Black Gate of their land; and there now their Lord was gathering in haste great forces to meet the onslaught of the Captains of the West. Upon the out-thrust spurs forts and towers were built, and watch-fires burned; and all across the gap an earth-wall had been raised, and a deep trench delved that could be crossed only by a single bridge."
From The Return of the King, LoTR Book 6, Ch 2, The Land of Shadow

"At the north end of the Ephel Duath, at its junction with the Ered Lithui, lay a deep circular valley ringed by sheer black barren cliffs - Udûn. Tolkien described the vale as being encircled by arms sent out by the two ranges. The symmetry of the valley suggests either a caldera or a ring-dike. A caldera is the remnant of a volcano that has exploded and/or collapsed. A ring-dike is a circular ridge of cooled igneous rock surrounding a deep valley. It occurs when a round block subsides into an underlying magma chamber and the fluid magma is forced up around the edges. Often the upwelling is intermittent, leaving passes such as the Isenmouthe and Cirith Gorgor. Either process could have resulted in the feature shown by Tolkien, although compared with our world either would have been gigantic. Imagine the original height of a volcano with a forty-five mile base. This colossus would have towered almost 29,000 feet! In contrast, Mt. Doom was only seven miles across and stood 4500 feet."
From The Atlas of Middle-earth, Revised Edition, by Karen Wynn Fonstad, Section 4, Regional Maps: Mordor and Adjacent Lands